Notch filter and apparatus for receiving and transmitting radio-frequency signals incorporating same

ABSTRACT

A notch filter suitable for attenuating certain frequencies of a radio-frequency signal includes an input for receiving the radio-frequency signal and an output for the output of a portion of the radio-frequency signal, first and second capacitive means, at least one inductor and a negative resistance circuit suitable for compensating the resistive losses of said at least one inductor. The inductor and the first and second capacitive means are placed to produce a resonator and the filter comprises a control device suitable for controlling the negative resistance circuit. The input impedance of the filter comprises a pole and a zero, with the pole depending on the second capacitive means and the zero depending on both the first and second capacitive means. The first and second capacitive means are variable and the control device is suitable for controlling the first and second capacitive means.

RELATED APPLICATION

The present application claims priority of European Patent ApplicationNo. 05425715.9 filed Oct. 12, 2005, which is incorporated herein in itsentirety by this reference.

FIELD OF THE INVENTION

The present invention herein refers to a notch filter, in particular toa filter that operates in radio-frequency, and to an apparatus forreceiving and transmitting radio-frequency signals.

BACKGROUND OF THE INVENTION

Currently, with the development of wireless communications, there is ahigh market demand for receiver-transmitter devices suitable fortransmitting and receiving a large quantity of data and equipped with awide frequency band. Such transceivers must be integrated in a chip ofsemi-conductor material. For this reason various types of receivers likethose with direct conversion have been produced that enable the problemsin the traditional super heterodyne receivers to be overcome. The latterpreferably comprise a band-pass filter, a low noise amplifier, a notchfilter or notch filter and a mixer.

The sensitivity of the receivers can be degraded by spurious out-of-bandsignals due to various mechanisms. In superheterodyne receivers, theimage signal has the same frequency deviation as the desired signal inregard to the clock of the local oscillator and has to be rejected. Thisoperation can be facilitated through the use of a notch filter withprogrammable band, as the frequency of the image signal is known.

In some transceivers the signal transmission and reception circuits worksimultaneously; the power of the transmitted signal is much higher thanthe power of the received signal and the insulation of the transmittedsignal from the received signal is limited at the radio frequencies. Inparticular, if the receiver circuit is the type with direct conversion,its sensitivity can be greatly reduced by the lost fraction or leakageof the signal transmitted that overlaps the signal received. Consideringthe frequency of the transmitted signal, which can vary in accordance tothe selected channel, is known, the rejection of the transmitted signalcan be improved using a notch filter with programmable band.

Currently there are various transceiver devices that comprise a notchfilter. However, such circuitry presents several inconveniences linkedto the presence of noise during signal reception and to a limitedaccuracy of the frequency calibration.

The most common solution adopted to lessen the losses of the transmittedsignal is to attenuate them at the input of the receiver or after thefirst amplification stage by using radio-frequency filters. Seeing theoffset between the reception and transmission frequencies is rather low,the filters to use must be highly selective. These filters cannot beintegrated into the same chip of the transceiver device, and thepresence of the filters outside the chip of the transceiver device makesthe solution very expensive, with a high consumption of energy.

In US Patent Application Publication No. 2004/0219900, a notch filterwith high Q factor is described which is integrated with a first and asecond low-noise amplifier and is totally contained in an integratedchip; said filter is suitable for the image rejection in radio-frequencyapplications. The notch filter comprises two Q factor enrichmentcircuits that are combined together to generate a negative impedancesuch that it compensates the losses in the low Q factor inductorslocated on the chip. To improve the image rejection the notch filteruses a circuit of automatic calibration of the current that consists ofan analog multiplier.

SUMMARY OF THE INVENTION

In view of the state of the technique described, an object of thepresent invention is to provide a notch filter that is more accuratethan the known filters and that permits lower consumption of energy.

In accordance with the present invention, this object is achieved bymeans of a notch filter suitable for attenuating certain frequencies ofa radio-frequency signal, the filter including an input for receivingsaid radio-frequency signal and an output for the output of a portion ofsaid radio-frequency signal, first and second capacitive means, at leastone inductor and a negative resistance circuit suitable for compensatingthe resistive losses of said at least one inductor. The inductor andfirst and second capacitive means are arranged to produce a resonator.The filter further includes a control device for controlling thenegative resistance circuit. The input impedance of the filter comprisesa pole and a zero, with the pole depending on the second capacitivemeans and the zero depending on both the first and second capacitivemeans. The first and second capacitive means are variable and thecontrol device is suitable for controlling said first and secondcapacitive means.

In an embodiment of the present invention, the notch filter can beregulated digitally so that the same filter can be adapted totime-varying signals. The filter is less selective than the knownfilters and thus can be integrated into the same chip of a transceiverapparatus.

The notch filter of the present invention may be incorporated in anapparatus for receiving and transmitting radio-frequency signals. Theapparatus includes an antenna, a transmitter, a receiver, a duplexeroperative for switching the signal to be transmitted towards the antennaand the signal received with the antenna towards the receiver. Thereceiver includes a first and second low-noise amplifiers and a notchfilter of the present invention placed between the first and secondamplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the present invention will appearevident from the following detailed description of an embodimentthereof, illustrated as non-limiting example in the enclosed drawings,in which:

FIG. 1 is a block diagram of an apparatus for receiving and transmittingsignals, wherein said receiving and transmitting apparatuses operatesimultaneously;

FIG. 2 is a block diagram of the notch filter according to the presentinvention;

FIG. 3 is a diagram of an array of capacitors of the circuit of FIG. 2;

FIG. 4 is a diagram of the input impedance as a function of thefrequency;

FIG. 5 is a circuit that implements a linear dependence of thetrans-conductance gain on a digital word; and

FIG. 6 is a more detailed circuit diagram of a part of the filter ofFIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows a diagram of an apparatus for receiving and transmittingradio-frequency signals comprising a notch filter according to thepresent invention. Said notch filter 1 is normally placed between afirst amplifier 2 coupled to the radio-frequency input signal IN and asecond amplifier 3 whose output signal OUT represents theradio-frequency output signal. The notch filter 1 belongs to thereception-transmission apparatus of radio-frequency signals; in saidapparatus the radio-frequency input signal IN comprises the loss signalor leakage of the transmitted signal TX and said signal must beattenuated by the filter, as can be seen in FIG. 1. The signal receivingand transmitting apparatus comprises an antenna 101 for receiving thesignal RX and for transmitting the signal TX, a transmitter 103 and areceiver 104, a duplexer 100 suitable for switching the transmittedsignal TX towards the antenna and the signal RX towards the receiver104; the latter comprises the amplifier 2, the filter 1 and the secondamplifier 3. The signal IN therefore comprises the received signal RXand the loss signal of the transmitted signal TX.

The filter 1 is of programmable type and comprises a device 4 forcalibrating the frequency and the Q factor, as can be seen in FIG. 2.The device 4 enables both the central frequency of the filter and the Qfactor to be measured and adjusted without disconnecting the othercircuits during the calibration phase. The filter comprises digitalcomponents that can be programmed to improve the linearity and to permitgreater flexibility in controlling the frequency of the filter. Thedevice 4 is capable of programming the filter to put it intoself-oscillating conditions and to measure the frequency and theamplitude of said oscillation. By means of the relations between theoscillation frequency and the central frequency and between theoscillation amplitude and the Q factor the filter can be calibrated.

The filter 1 is coupled to the amplifier 2 so that the output currentIout of the amplifier 2 is subdivided into a current Ifilt in input tothe filter 1 and a current Iin in input to the amplifier 3. The filter 2comprises an inductor L coupled between input terminal A of the filterand ground GND, an array of programmable capacitors C1(n1) coupledbetween the terminal A and a terminal of the inductor L, a resistor R1,that keeps track of the losses due to the array C1(n1), placed betweenthe inductor and the array C1(n1), an array of programmable capacitorsC2(n2) placed in parallel to the inductor L, a resistance R2 placed inparallel to the inductor L and suitable for keeping track of the lossesof the parallel between L and C2(n2), and a negative resistance circuit5 comprising a programmable transconductance amplifier Gm in positivefeedback configuration having the task of cancelling the resistance R2.A possible implementation of a programmable array is shown in FIG. 3where a fixed value capacitor Co is placed in parallel to N circuitbranches, comprising a switch S and a variable capacitor with the valueClsb for the first branch (or branch 1) at the value 2^(N-1)Clsb for thelast branch. In this case the array is controlled by a digital binaryword and Clsb is the value of the capacitor with the least significantbit.

The output B of the transconductance amplifier Gm is in input to thedevice 4 suitable for programming the arrays C1(n1) and C2(n2) and thetransconductance amplifier Gm. The device 4 comprises a peak detector 44suitable for detecting the peak of the signal on the terminal B, anamplitude comparator 41 suitable for comparing the value of theamplitude of the signal coming from the peak detector 44 with the valueof a reference voltage Vref, a frequency comparator 42 suitable forcomparing the frequency of the signal on the terminal B with a referencefrequency fref and a digital controller 43 having in input the outputsof the comparators 41 and 42 and sending in output the digital signalsn1, n2 and m suitable for programming the arrays C1(n1) and C2(n2) andthe transconductance amplifier Gm.

The calibration of the frequency comes about by increasing the value ofthe transconductance Gm to determine the oscillation of the componentsL, C1 and C2. The oscillation frequency is compared with the referencefrequency fref by means of the comparator 42 and the value of thecapacitive arrays C1 and C2 is adjusted by means of the digital data n1and n2.

The calibration of the Q factor is done by comparison of the amplitudeof the oscillations with the value of the voltage Vref; the value of thetransconductance gain Gm is adjusted by means of the control word m.

Considering the resistance R2 exactly compensated the input impedance ofthe filter Zf has a localized pole-zero couple at the following angularfrequencies:

$\omega_{z} = {{\frac{1}{L\left( {{C\; 1\left( {n\; 1} \right)} + {C\; 2\left( {n\; 2} \right)}} \right)}\mspace{14mu}{and}\mspace{14mu}\omega_{p}} = {\frac{1}{L*C\; 2}.}}$The input impedance of the filter is R1 at the angular frequency ω_(z);in this manner, given that the value of R1 is conceived to be negligibleif compared with the value of the resistance Ro of the amplifier 3, thecurrent Ifilt is much greater than the current Iin while the contrarycomes about at the angular frequency ω_(p).

The angular frequency ω_(z) is the frequency of maximum attenuation ofthe notch filter 1 and corresponds to the frequency of the transmittedsignal while the angular frequency ω_(p) determines the bandwidth of thefilter. Said two angular frequencies can be programmed independently ofeach other by means of the control device 4. This permits highflexibility in controlling the frequency response of the filter. In factduring the phase of calibration of the angular frequencies ω_(p) andω_(z) the ratio ω_(p)/ω_(z) must not be changed for compensating theprocess variations and therefore the calibration of both the digitaldata n1 and n2 is necessary. Once the frequency ω_(p) that depends onlyon the capacitor C2(n2) is fixed, only the digital data n1 could beadjusted to change the ratio ω_(p)/ω_(z); in this manner it is possibleto program the frequency of the notch filter independently from thefrequency ω_(p).

The calibration procedure comes about in the following manner. Initiallythe calibration of the gain Gm comes about. The value of thetransconductance gain Gm is increased to start up the oscillation of theparallel resonator composed of three branches: the inductor L, thecapacitance C2(n2) and the branch composed of the resistances Ro and R1and of the capacitance C1(n1). The datum or digital word m, that setsthe value of the transconductance gain Gm, is regulated by detecting theamplitude of the oscillation and comparing it with the value of thereference voltage Vref. Said operation is carried out by means of thepeak detector 44, the comparator 41 and the digital controller 43.

The latter comprises a processor suitable for carrying out the activityof the digital controller 43 by means of the following operations.During the filter calibration phase the processor searches among thedigital codes n1, n2 and m those that enable the frequency and theoscillation amplitude desired to be obtained. The calibration phase canbe made only for the central transmission frequency or alternativelyrepeated for every possible transmission frequency. The final code isstored in an internal memory to be accessible in the following phases.During the normal operation of the filter, the processor processes thecorrect codes n1, n2 and m to supply them to the filter. Said processingis based on the stored calibration codes and on the transmitted channelcode that is supplied in input to the processor.

The oscillation frequency is compared with the reference frequency frefand the capacitive arrays C1 (n1) and C2 (n2) are adjusted in accordancewith the result of the comparison, until the frequency error is mademinimum. Considering that it is necessary to impose the ratioω_(p)/ω_(z) between the angular frequencies of the pole and of the zeroconstant, the datum or digital word for controlling the capacitances C1and C2 must be equal, that is n1=n2=n. Said operations are carried outby the comparator 42 and by the digital controller 43.

The oscillation frequency coincides with the central frequency of thefilter; once the oscillation frequency is fixed the pole and the zero ofthe filter, that is the band-pass and the attenuation band of the filter1, are also fixed. However, after the filter calibration phase, it ispossible to vary only the digital word n1 to vary the position of thezero, should transmitted channel variations be present that move thefrequency of the loss signal of the transmitted signal.

FIG. 4 shows the trend of the input impedance of the filter Zf inabsolute value in function of the angular frequency; it can be seen fromthe graph that the impedance Zf has the minimum coinciding with the zeroω_(z) and the maximum coinciding with the pole ω_(p).

Considering the circuit of FIG. 2, the equivalent capacitance C1 p ofthe series of components Ro, R1 and C1 can be calculated. We have

${C\; 1p} = \frac{C\; 1}{1 + \left\lbrack {{\omega_{z}\left( {{Ro} + {R\; 1}} \right)}C\; 1} \right\rbrack^{2}}$which is slightly lower than the value of C1. This determines an errorfor the frequency associated to the zero given that:

${\frac{\Delta\;{fz}}{fz}} = {{\frac{1}{2}\frac{1}{1 + {C\;{2/C}\; 1}}{\frac{{C\; 1} - {C\; 1p}}{C\; 1}}} \approx {\frac{1}{2}{{\frac{1}{1 + {C\;{2/C}\; 1}}\left\lbrack {{\omega_{z}\left( {{Ro} + {R\; 1}} \right)}C\; 1} \right\rbrack}^{2}.}}}$

Therefore the error on the frequency associated to the zero can be madesmall by reducing the time constant (R0+R1)C1.

Calculating the equivalent resistance R1 p of the series of the elementsRo, R1 and C1 we have:

R1 p≈(Ro+R1)[ω_(z)(Ro+R1)C1]⁻² and therefore to keep the oscillation thetransconductance gain must be Gmosc=−(R2+R1 p)/(R2*R1 p) where Gmosc isthe transconductance gain in oscillation conditions. The ratio r betweenthe transconductance gain Gmosc and the transconductance gain of thefilter Gm=−1/R2 must be:

$r \approx {{\frac{R\; 2}{{Ro} + {R\; 1}}\left\lbrack {{\omega_{z}\left( {{Ro} + {R\; 1}} \right)}C\; 1} \right\rbrack}^{2} + 1.}$

In typical applications r varies between 1.5 and 2. The ratio r ismainly subject to variations of the Q factor of the inductor, of theinput impedance of the second amplifier Ro and of the capacitance C1;said variations can be controlled satisfactorily permitting an accuracyof the ratio r equal to 10%.

To find the word m that controls the transconductance gain Gm, theamplitude of the oscillation must be made equal to the value Vref. Thelatter must be equal to the amplitude of the signal in input to thetransconductance amplifier when the input current Ifilt is at themaximum value during the normal operation of the filter. This enablesthe ratio r also found in high-level signal conditions to be made valid.

The ratio “r” can be simply implemented by a digital control circuit ifthe dependence of the transconductance gain Gm on the word m is linear.A circuit that produces said linear dependence of Gm on m is shown inFIG. 5. A transconductance amplifier 301 has a gain given by Gm/K whereK is a constant. The amplifier 301 is coupled to ground GND and has ininput the voltage Vref; the output of the amplifier 301 is the input ofan error amplifier 300 at high gain which, by means of the voltageVbias, forces the output current of the amplifier 301 to be equal toIdac, that is the output current of a digital-analog converter 302controlled by the word m. The reference voltage Vref is the outputvoltage of an operational amplifier 303 having in input on the invertingterminal the current Ir, the non-inverting terminal coupled to groundand the output terminal coupled to the inverting terminal by means of aresistance R. The digital-analog converter 302 has the reference currentIr in input and the terminal on which the current Idac flows is coupledto ground GND by means of the resistance Rc. We have:

${Idac} = {{{Vref}\;\frac{G\; m}{K}} = {R \times {Ir} \times {\frac{G\; m}{K}.}}}$If Idac=m*Ir we have

${G\; m} = {m\;{\frac{K}{R}.}}$

The voltage Vbias is applied to a replica of the circuit 301 that has tobe K times greater and comprises circuit 5 of FIG. 2.

A circuit implementation of a part of the notch filter of FIG. 1 isshown in FIG. 6. The negative impedance circuit 5 is produced by meansof a couple of differential bipolar cross-coupled transistors Q1 and Q2;the cross coupling produces the positive feedback needed to generate anegative impedance. The emitter terminals of the transistors Q1 and Q2are coupled to a current generator It(m), coupled in turn to ground GND,that can depend directly on the digital word m or indirectly by means ofthe circuit described in FIG. 5. Both the inductor L and the capacitorC1(n1) are divided respectively into two inductors of the value L/2 andtwo capacitors of the value 2*C1(n1), while the parasitic resistances R1and R2 are also halved. The common terminal of the inductors L/2 iscoupled to a positive supply for the polarization of the circuit 5. Theoutput voltage of the parallel resonator formed by the branches L,C2(n2), and the series of the components Ro, R1 and C1(n1) must beconsidered as a differential output voltage between the nodes B+ and B−which is in input to the control device 4.

While there have been described above the principles of the presentinvention in conjunction with specific memory architectures and methodsof operation, it is to be clearly understood that the foregoingdescription is made only by way of example and not as a limitation tothe scope of the invention. Particularly, it is recognized that theteachings of the foregoing disclosure will suggest other modificationsto those persons skilled in the relevant art. Such modifications mayinvolve other features which are already known per se and which may beused instead of or in addition to features already described herein.Although claims have been formulated in this application to particularcombinations of features, it should be understood that the scope of thedisclosure herein also includes any novel feature or any novelcombination of features disclosed either explicitly or implicitly or anygeneralization or modification thereof which would be apparent topersons skilled in the relevant art, whether or not such relates to thesame invention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as confronted by thepresent invention. The applicant hereby reserves the right to formulatenew claims to such features and/or combinations of such features duringthe prosecution of the present application or of any further applicationderived therefrom.

1. A notch filter suitable for attenuating certain frequencies of aradio-frequency signal, said filter comprising: an input for receivingsaid radio-frequency signal and an output for the output of a portion ofsaid radio-frequency signal; first and second capacitive means; at leastone inductor and a negative resistance circuit for compensating theresistive losses of said at least one inductor, said inductor and saidfirst and second capacitive means being placed to produce a resonator;and a control device suitable for controlling said negative resistancecircuit, the input impedance of said filter comprising a pole and azero, wherein said pole depends on said second capacitive means and saidzero depends on both said first and second capacitive means, said firstand second capacitive means are variable and said control device isoperative for controlling said first and second capacitive means,wherein said control device comprises: means for detecting the peak ofsaid portion of signal in output from the filter; an amplitudecomparator for comparing the value of the signal in output from the peakdetector with a reference signal; and a frequency comparator forcomparing the frequency of the portion of signal in output from thefilter with a reference frequency, said comparators being operative forsending signals representative of said comparisons to a logic meansoperative for sending control signals to said first and secondcapacitive means and to said negative resistance circuit until theamplitude and the frequency of said portion of signal in output from thefilter are substantially equal to said reference signal and to saidreference frequency.
 2. The notch filter according to claim 1, whereinsaid control device is operative for controlling said first and secondcapacitive means independently from each other.
 3. The notch filteraccording to claim 1, wherein said control device is operative forvarying said first capacitive means to vary the frequency associated tosaid zero.
 4. The notch filter according to claim 1, wherein said firstand second capacitive means can be controlled digitally and said controldevice comprises digital control means.
 5. The notch filter according toclaim 1, wherein said first and second capacitive means are formed by adigitally programmable array of capacitors.
 6. An apparatus forreceiving and transmitting radio-frequency signals comprising anantenna, a transmitter and a receiver, a duplexer for switching thesignal to be transmitted towards the antenna and the signal received bymeans of the antenna towards the receiver, said receiver comprising afirst low-noise amplifier, a second low-noise amplifier and a notchfilter placed between said first and said second amplifier, said notchfilter comprising: an input for receiving said radio-frequency signaland an output for the output of a portion of said radio-frequencysignal; first and second capacitive means; at least one inductor and anegative resistance circuit for compensating the resistive losses ofsaid at least one inductor, said inductor and said first and secondcapacitive means being placed to produce a resonator; and a controldevice suitable for controlling said negative resistance circuit, theinput impedance of said filter comprising a pole and a zero, whereinsaid pole depends on said second capacitive means and said zero dependson both said first and second capacitive means, said first and secondcapacitive means are variable and said control device is operative forcontrolling said first and second capacitive means, wherein said controldevice of the notch filter comprises: means for detecting the peak ofsaid portion of signal in output from the filter; an amplitudecomparator for comparing the value of the signal in output from the peakdetector with a reference signal; and a frequency comparator forcomparing the frequency of the portion of signal in output from thefilter with a reference frequency, said comparators being operative forsending signals representative of said comparisons to a logic meansoperative for sending control signals to said first and secondcapacitive means and to said negative resistance circuit until theamplitude and the frequency of said portion of signal in output from thefilter are substantially equal to said reference signal and to saidreference frequency.
 7. The apparatus according to claim 6, wherein saidfirst amplifier comprises an output terminal and said second amplifiercomprises an input terminal, said notch filter is stably coupled to theoutput terminal of the first amplifier and to the input terminal of thesecond amplifier.
 8. The apparatus according to claim 6, wherein saidfilter and said first and second amplifier can be integrated into thesame semiconductor chip.
 9. The apparatus of claim 6, wherein saidcontrol device of the notch filter is operative for controlling saidfirst and second capacitive means independently from each other.
 10. Theapparatus of claim 6, wherein said control device of the notch filter isoperative for varying said first capacitive means to vary the frequencyassociated to said zero.
 11. The apparatus of claim 6, wherein saidfirst and second capacitive means of the notch filter can be controlleddigitally and said control device comprises digital control means. 12.The apparatus of claim 6, wherein said first and second capacitive meansof the notch filter are formed by a digitally programmable array ofcapacitors.
 13. A notch filter suitable for attenuating certainfrequencies of a radio-frequency signal, said filter comprising: aninput for receiving said radio-frequency signal and an output for theoutput of a portion of said radio-frequency signal; first and secondcapacitors; at least one inductor and a negative resistance circuit forcompensating the resistive losses of said at least one inductor, saidinductor and said first and second capacitors being placed to produce aresonator; and a control circuit for controlling said negativeresistance circuit, the input impedance of said filter comprising a poleand a zero, wherein said pole depends on said second capacitor and saidzero depends on both said first and second capacitors, said first andsecond capacitors being variable, wherein said control circuit controlssaid first and second capacitors, and wherein said control circuitcomprises: a peak detector for detecting the peak of said portion ofsignal output from the filter; an amplitude comparator for comparing thevalue of the signal output from the peak detector with a referencesignal; and a frequency comparator for comparing the frequency of theportion of signal output from the filter with a reference frequency,said comparators being operative for sending signals representative ofsaid comparisons to a logic circuit operative for sending controlsignals to said first and second capacitors and to said negativeresistance circuit until the amplitude and the frequency of said portionof signal output from the filter are substantially equal to saidreference signal and to said reference frequency.
 14. The notch filteraccording to claim 13, wherein said control circuit controls said firstand second capacitors independently from each other.
 15. The notchfilter according to claim 13, wherein said control circuit varies saidfirst capacitor to vary the frequency associated with said zero.
 16. Thenotch filter according to claim 13, wherein said first and secondcapacitors can be controlled digitally and said control circuitcomprises a digital control circuit.
 17. The notch filter according toclaim 13, wherein said first and second capacitors are formed by adigitally programmable array of capacitors.
 18. A notch filtercomprising: a pair of cross-coupled transistors having a first node, asecond node, and a third node coupled to a current source; a firstcapacitor circuit coupled between the first node and an input; a secondcapacitor circuit coupled between the second node and ground; a thirdcapacitor circuit coupled across the first and second nodes; and aninductor circuit coupled across the first and second nodes, wherein thefirst and second nodes form a differential output.
 19. The notch filterof claim 18 wherein the current source is digitally controlled.
 20. Thenotch filter of claim 18 wherein the inductor circuit is coupled to asource of supply voltage.